1. Field of the Invention
The present invention relates generally to a torque converter used for an automatic transmission or the like. More particularly, it pertains to a lockup control system which properly performs a lockup releasing control when effecting changeover from a lockup mode in which input and output elements of the torque converter are directly coupled to each other, to a converter mode in which the direct coupling between the input and output elements is released.
2. Description of the Related Art
Generally, automatic transmission inclusive of continuously variable transmission comprises a torque converter on its input side, to achieve torque multiplication or absorb torque fluctuation. As generally known in the art, ordinary torque converter suffers from a low power transmission efficiency because power is transmitted from the input element to the output element via a working fluid in the converter. Thus, it is a recent trend to employ a lockup-type torque converter comprising a lockup clutch which is capable of directly coupling the input and output elements when torque multiplication function and/or torque fluctuation absorption function are not required.
The above-mentioned lockup clutch for directly coupling the input and output elements is engaged by a differential pressure across the clutch, and disengaged to release the direct coupling or lockup as the differential pressure is reduced. In order to minimize a shock upon releasing the lockup, Japanese Patent Application Laid-Open Publication No. 2-35,266 discloses a technology wherein the differential pressure across the lockup clutch is reduced at a predetermined variation rate depending upon a selected speed range of the automatic transmission.
With such a conventional lockup releasing technology, however, not only does the operation give somewhat unnatural feel in that the time required for the lockup releasing becomes different depending upon the speed range of the transmission, but also there arises a problem as follows. That is to say, when achieving a lockup state by engaging the lockup clutch and thereby directly coupling the input and output elements to each other, it has been a conventional practice from the viewpoint of facilitating the control to maximize the differential pressure across the lockup clutch, without performing a pressure control. This means that, for releasing the lockup, the differential pressure has to be gradually reduced at the above-mentioned predetermined variation rate from the maximum differential pressure. In this instance, the reduction of the differential pressure at the predetermined rate is performed even in a pressure rage from the maximum differential pressure to a certain pressure level which is not relevant to the lockup releasing shock, thereby unnecessarily prolonging the time required for releasing the lockup.
In view of such a problem, there has been proposed an improved lockup releasing control technology which is actually employed in A32-type passenger cars manufactured and sold as from 1994 under the trade name "Nissan Maxima" by Nissan Motor Co., Ltd., the assignee of this application, and which will be explained below with reference to FIG. 9. This technology is featured by a unique reduction control of the differential pressure P.sub.A -P.sub.R between the torque converter apply pressure P.sub.A and a torque converter release pressure P.sub.R on opposite sides of the lockup clutch. More particularly, as shown in FIG. 9, the differential pressure P.sub.A -P.sub.R is instantaneously reduced at the instant t.sub.1 of the lockup releasing command from the maximum differential pressure to a certain initial differential pressure .DELTA.P.sub.0 which is not relevant to the lockup releasing shock, and is then gradually reduced at a constant variation rate to the minimum differential pressure. With such a reduction control, the lockup clutch is disengaged and the lockup is released at the instant t.sub.2 when the differential pressure P.sub.A -P.sub.R becomes zero, and the control is completed at the instant t.sub.3 when the differential pressure P.sub.A -P.sub.R reaches the minimum differential pressure.
The above-mentioned lockup releasing control technology which is actually employed in "Nissan Maxima" proved to be highly advantageous for shortening the time required for releasing the lockup, though it would be still desirous to further improve the technology by refining the control function.